Systems and methods using tunable differential gravimeter

ABSTRACT

Systems and methods are provided for determining a property, e.g., density, of a geological formation based on Einstein&#39;s theory of gravitation. A gravitational potential difference is determined between two positions of the geological formation by measuring a frequency shift of radiation travelling from a source to an absorber of a differential gravimeter. The differential gravimeter can be a part of a downhole tool. The gravitational potential difference determined can be used to determine the property of the geological formation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/578,846, filed Dec. 21, 2011.

BACKGROUND

Gravimetry is a type of measurement that has been used for reservoircharacterization and gas-movement monitoring. Some commerciallyavailable gravimeters have been based on Newton's theory of gravitation.For example, Lacoste-Romberg gravimeters use a zero-length spring tomonitor the gravitational force on a small test mass. Some gravimetersmonitor the time of flight of a free-falling object or the oscillationtime of a pendulum. More recently, some gravimetry measurements monitorthe position of a superconducting niobium sphere suspended by a magneticfield.

In contrast to Newton's theory of gravitation, Einstein's theory ofgravitation introduces a general relationship between a geometricstructure of space-time and the presence of massive bodies. Einstein'stheory predicts a time dilatation in the presence of a massive body.Such time dilatation is referred as “Gravitational Red Shift”.

Nuclear spectroscopy offers a process that has been used to observe andconfirm the gravitational red shift predicted by Einstein's theory ofgravitation. For example, the isotope ⁵⁷Fe nucleus can emit from itslowest excited state a 14.4 keV photon. This state can have anapproximate lifetime of 140 nanosecond (ns), a spectral-line width ofabout 10⁻⁸ eV or a relative line width of about 9.2×10⁻¹³. The Mossbauereffect recognizes that a nuclear transition has a high frequencyprecision, or narrow spectral line width, such as that of the ⁵⁷Fenucleus. The Mossbauer effect has successfully been used tomeasure/verify the gravitational red shift, as predicted by Einstein'stheory of gravitation. Pound and Rebka in 1960 were the first to use aMossbauer experiment to quantitatively confirm the gravitational redshift of Einstein's theory of gravitation.

SUMMARY

Generally, systems, methods and techniques are described that aredirected to determining properties of a geological formation from agravitational potential difference, by measuring a frequency shift ofradiation resulting from the gravitational potential difference. As oneexample, the density of a geological formation may be determined.

In one embodiment, a system for determining a property of a geologicalformation is provided. The system includes a component contained in adownhole tool. The component measures a frequency shift of gammaradiation travelling from a first position of the component to a secondposition of the component. The frequency shift is resulting from agravitational potential difference between the first position and thesecond position, which correspond to respective positions of thegeological formation.

The component includes a source positioned at the first position. Thesource emits gamma radiation from the first position to the secondposition. An absorber is positioned at the second position to absorb thegamma radiation or a portion of the gamma radiation emitted from thesource. A tuning mechanism is operatively connected to the source or theabsorber or both to compensate the frequency shift and induce a resonantabsorption of the gamma radiation emitted from the source by theabsorber. A detector or detectors are operatively connected to theabsorber. The detector(s) detect the absorption of the gamma radiationby the absorber and generate a signal or signals associated with thefrequency shift. A processor or processors are operatively connected tothe detector(s) to receive the signal or signals generated, determinethe gravitational potential difference from the signal or signalsreceived, and determine the property of the geological formation betweenthe first and second positions based on the gravitational potentialdifference determined.

In another embodiment, a method is provided for determining a propertyof a geological formation. The method includes generating gammaradiation at a first position of the geological formation, and emittingthe gamma radiation from the first position to a second position of thegeological formation. The gamma radiation travelling from the firstposition to the second position has a frequency shift resulting from agravitational potential difference of the geological formation betweenthe first and second positions. The method further includes compensatingthe frequency shift of the gamma radiation to induce a resonantabsorption of the gamma radiation, and absorbing the gamma radiation ora portion of the gamma radiation at the second position of thegeological formation. The method further includes detecting theabsorption of the gamma radiation and generating a signal or signalsassociated with the frequency shift, determining the gravitationalpotential difference based on the signal or signals generated, anddetermining the property of the geological formation between the firstand second positions based on the gravitational potential differencedetermined.

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of an example system describedherein.

FIG. 2 illustrates a schematic diagram of one embodiment of a systemdescribed herein including an instrument, for example, a wireline toolsuspended in a wellbore.

FIG. 3 illustrates a schematic diagram of one embodiment of anothersystem described herein including an instrument, for example, ameasurement-while-drilling (MWD) or logging-while-drilling (LWD) tooldisposed in a wellbore.

FIG. 4 illustrates a schematic diagram of an example component thatincludes a differential gravimeter described herein with a mechanicalmodulator for compensating the frequency shift of a radiation from asource.

FIG. 5 a illustrates a schematic diagram of another example componentthat includes a differential gravimeter described herein with a magneticmodulator for compensating the frequency shift of a radiation from asource.

FIG. 5 b illustrates an example of Zeeman splitting of a spectral linewith four-fold degeneracy (j=3/2).

FIG. 6 illustrates a flow diagram of one embodiment for determining aproperty of a geological formation.

FIG. 7 illustrates a flow diagram of one embodiment for determining adensity of a geological formation.

FIG. 8 illustrates a schematic diagram of an example of a concentricspherical shell model.

DETAILED DESCRIPTION

Systems, methods and techniques are described herein for determining aproperty of a geological formation based on Einstein's theory ofgravitation.

Einstein's theory predicts a time dilatation in the presence of amassive body. For example, in the presence of the earth, the frequency fof radiation at two positions of a geological formation can be differentif there is a gravitational potential difference between these twopositions. The radiation can be, for example, a nuclear spectral line ata certain energy or frequency. For example, the nucleus of isotope ⁵⁷Fecan emit a 14.4 keV photon with a spectral-line width of about 10⁻⁸ eV.At a first position of the geological formation, the nuclear spectralline of the radiation has a frequency f₁ and at a second position of thegeological formation, the frequency can change to be f₂. The frequencydifference δf=f₁−f₂ can be expressed as:

$\begin{matrix}{{\delta\; f} = {{f_{1} - f_{2}} = {\left\lbrack {\sqrt{1 - {\frac{2}{c^{2}}{\Phi\left( {M,r_{1}} \right)}}} - \sqrt{1 - {\frac{2}{c^{2}}{\Phi\left( {M,r_{2}} \right)}}}} \right\rbrack f_{0}}}} & (1)\end{matrix}$where r₁ is the radius of the first position of the geologicalformation; r₂ is the radius of the second position of the geologicalformation; Φ(M, r₁) is the gravitational potential at the firstposition; Φ(M, r₂) is the gravitational potential at the secondposition; f₀ is the frequency of the radiation in the absence of anyheavy body; c is the speed of light in vacuum; and M is the mass of theearth.

An approximation of Equation (1) can be

$\begin{matrix}{{\delta\; f} = {{f_{1} - f_{2}} \cong {\left\lbrack {{\Phi\left( {M,r_{1}} \right)} - {\Phi\left( {M,r_{2}} \right)}} \right\rbrack\frac{f_{0}}{c^{2}}}}} & (2)\end{matrix}$Equation (1) or (2) illustrates the relationship between a frequencyshift and a gravitational potential difference.

A gravitational potential difference, such as δΦ(M, r₁)=[Φ(M, r₁)−Φ(M,r₂)], can be determined from a measured frequency shift, such asδf=f₁−f₂, by

$\begin{matrix}{{{\delta\Phi}\left( {M,r_{1}} \right)} = {\left\lbrack {{\Phi\left( {M,r_{1}} \right)} - {\Phi\left( {M,{r_{1} - {\delta\; r}}} \right)}} \right\rbrack \cong {2\frac{f_{1} - f_{2}}{f_{1} + f_{2}}c^{2}}}} & (3)\end{matrix}$where δr=r₁−r₂ is a vertical distance between the first and secondpositions. The vertical distance described herein refers to a distancein the direction of a gravitational force.

A property of the geological formation, e.g., the average formationdensity between the first and second positions, can be determined fromthe gravitational potential difference determined, which will bedescribed further below.

FIG. 1 illustrates an example system 100 for determining a property of ageological formation. The system 100 can be associated with aninstrument, e.g., a downhole tool, but may not be limited to a downholetool. The system 100 includes a component 110 and at least one processor108 operatively connected to the component 110. The component 110 canhave a differential gravimeter that includes a source 102 to generateradiation, an absorber 104 to absorb the radiation from the source 102,and a tuning mechanism 106. The processor 108 is operatively connectedto the component 110 for receiving and/or sending signal(s) from/to thecomponent 110 and processing the signal(s). The processor 108 caninclude one or more modules, e.g., 108 a, to help with processingmeasurements and/or signals.

The source 102 generates radiation, e.g., gamma radiation, emitted tothe absorber 104. The radiation travelling from the source 102 to theabsorber 104 can have a frequency shift resulted from a gravitationalpotential difference between the position of the source 102 and theposition of the absorber 104. This frequency shift can be compensated bythe tuning mechanism 106, which is operatively connected to at least oneof the source 102 and the absorber 104, such that the radiation can beresonantly absorbed by the absorber 104. Resonance absorption describedherein can refer to an absorption process where the frequency ofincoming photons to be absorbed agrees with a characteristic resonancefrequency of an absorbing material. For example, if the two frequenciesmatch within a spectral line width of the photons, the resonanceabsorption can occur. The probability of a resonance absorption processcan be measured by a resonance absorption cross section which can berelated to the wavelength of photons.

A detector 105 can be operatively connected to the absorber 104 todetect the absorption and generate one or more signals associated withthe frequency shift. The signal(s) are received and processed by theprocessor 108. The processor 108 may send control signals to thecomponent 110 to control its operation. It will be appreciated that oneor more processors can be used. The processor 108 can include one ormore modules, e.g., 108 a, further described below to help withprocessing measurements and/or signals.

The system 100 can be used to determine a property of a geologicalformation. In one embodiment, a density of the geological formation canbe determined by the system 100. It will be appreciated that otherproperties of the geological formation may be determined separate fromand/or in addition to the formation density, e.g., fluid compositions,the change of the formation density, and/or the gravitational potentialdifference(s). For example, the system 100 can be used to monitorchanges in fluid composition of the geological formation. The formationdensity may be influenced by, for example, fluid content, mineralogy,and/or lithology. A density change may be due to, for example, a changein fluid content such as, for example, from relative contents of gas andwater, and/or compaction of rock during depletion.

The source 102 and the absorber 104 can include at least one isotope,for example, ⁵⁷Fe, ⁶⁷Zn, ¹⁰⁹Ag, ¹⁰⁷Ag, ¹⁰³Rh, and ¹⁸⁹Os, etc. The term“isotope” described herein can include a nuclear isomer, which is, forexample, a metastable state of an atomic nucleus caused by theexcitation of one or more of its nucleons. Table 1 summarizes someproperties of example isotopes that may be employed. It is to beunderstood that other isotopes suitable for resonance absorption canalso be used. In one embodiment, the source 102 and the absorber 104 caninclude the same isotope(s), for example, the absorber 104 can be acrystal including the same decay material or isotope(s) as the source102.

TABLE 1 Properties of isotopes Energy Natural Line Recoilless Half lifeIsotope [keV] Lifetime Width [eV] Resolution Fraction of Parent ⁵⁷Fe14.4 140 ns  1.3 × 10⁻⁸  9.2 × 10⁻¹³ 0.7* 270 days ⁶⁷Zn 93.3 9.4 μs  1.4× 10⁻¹¹ 1.5 × 10⁻¹⁵ 0.0212 78 hr ¹⁰⁷Ag 93.5 63 s 1.05 × 10⁻¹⁷ 1.1 ×10⁻²² 0.0377 6.5 hr ¹⁰⁹Ag 88 57 s 1.16 × 10⁻¹⁷ 1.3 × 10⁻²² 0.0535 464days ¹⁰³Rh 39.75 81 min 1.35 × 10⁻¹⁹ 3.4 × 10⁻²⁴ 0.756 11.96 days ¹⁸⁹Os30.8 8.7 hr  2.1 × 10⁻²⁰ 6.8 × 10⁻²⁵ 0.905 13 days *at room temperature(about 300 K). Reference(s): A. Davydov, Hyperfine Interactions, 135(2001), 125-153.

In one embodiment, a beta-radioactive isotope of an element, such as oneof the isotopes listed in Table 1, can be included in the source 102and/or the absorber 104. The isotope can exist in, for example, amonocrystalline form, a ceramics form, or metals. The beta decay of theisotope can emit an associated photon from an excitation state with alifetime τ. The line width Γ=ℏ/τ(ℏ is Planck's constant) can be asnarrow as the line widths for example listed in Table 1, and agravimeter can have a resolution comparable to the resolutions listed inTable 1. As listed in Table 1, the values of the resolution can beobtained from the natural line width divided by the energy.

FIGS. 2 and 3 illustrate implementations of systems, for example, indownhole tools. In some embodiments, a downhole tool can incorporate thesystem 100 of FIG. 1. In other embodiments, a downhole tool can itselfbe the system 100 of FIG. 1. FIG. 2 illustrates a schematic diagram ofone embodiment of a system 200 including a component 220 for determininga property of a geological formation 202. The system 200 can determine agravitational potential difference between two positions of thegeological formation 202 from a measured frequency shift of radiationtraveling between the two positions, based on the relationshipdescribed, for example, in Equation (3). A property of the geologicalformation 202 can be determined from the gravitational potentialdifference determined. For example, a density of the geologicalformation 202 can be derived from the gravitational potentialdifference, which will be discussed further below. It will beappreciated that other properties of the geological formation 202, forexample, a formation compaction due to depletion, and/or changes influid composition of the geological formation such as changes in fluidcomposition from gas to water, may also be determined from thegravitational potential difference determined.

The system 200 includes an instrument 210 and a component 220 positionedwithin the instrument 210. The component 220 can be the component 110 ofFIG. 1. In this embodiment, the instrument 210 can be a downhole tool,for example, a wireline tool that can be suspended in a wellbore 204.The wellbore 204 can be vertical, more or less deviated from vertical oreven substantially horizontal. It is to be understood that theinstrument 210 is not limited to a wireline tool and can be ameasurement-while-drilling (MWD) tool, a logging-while-drilling (LWD)tool, a coiled tubing tool, a testing tool, a completions tool, aproduction tool, or combinations thereof, depending on the applications.For example, FIG. 3 illustrates ameasurement-while-drilling/logging-while-drilling tool disposed in awellbore which will be discussed below.

In one embodiment, as the downhole tool 210, runs over an extended depthinterval, within the wellbore 204, the tool 210 may serve to identifyand monitor density changes, possibly due to, for example, fluid frontsdisplacing gas or water encroaching in an asymmetrical fashion towardthe depth interval. The extended depth interval refers to, for example,the thickness of a hydrocarbon reservoir, possibly several tens ofmeters, which can be much larger than a measurement interval of the tool210. In one embodiment, the tool 210 can be used to determine within theextended depth interval the position of a formation-density change, forexample, the change from the low-density gas zone to the high-densitywater zone below, where the interface can change, for example, from yearto year, as gas is being produced-depleted.

A processor 230 is operatively connected to the component 220 forreceiving/sending signal(s) from/to the component 220 and processing thesignal(s). In this embodiment, the processor 230 is positioned withinthe downhole tool 210. In other embodiments, the processor 230 can bepositioned outside of the downhole tool 210. For example, FIG. 3illustrates a processor positioned above the wellbore, e.g., on thesurface. It is to be understood that one or more processors can be used.

As shown in FIG. 2 (and also FIG. 3), the downhole tool 210/310 ispositioned in a land based rig. It will be appreciated that the downholetool 210/310 can also be deployed in other ways such as, for example,from an offshore platform.

FIG. 3 illustrates a schematic diagram of one embodiment of anothersystem 300 including a component 320 for determining properties of ageological formation. Similar to the system 200 shown in FIG. 2, thesystem 300 includes an instrument 310, e.g., a downhole tool, and acomponent 320 positioned within the instrument 310. The component 320can be the component 110 of FIG. 1 or the component 220 of FIG. 2. Inthis embodiment, the instrument 310 is a downhole tool such as ameasurement-while-drilling (MWD) or logging-while-drilling (LWD) toolthat can be disposed in a wellbore 304 about a geological formation 302.

A processor 330 is operatively connected to the component 320 forreceiving/sending signal(s) from/to the component 320 and processing thesignal(s). In this embodiment, the processor 330 is positioned on thesurface. It will be appreciated that the processor 330 can be a part ofthe component 320 and/or on the downhole tool 310. It will be alsoappreciated that one or more processors can be used.

In the embodiments of FIGS. 2 and 3, the geological formation 202, 302is a subterranean formation penetrated by the wellbore 204, 304. It willbe appreciated that the geological formation 202, 302 does not have tobe a subterranean formation. The system 200, 300 can be applied forother geological formations generally above and/or below the ground. Forexample, the geological formation 202, 302 can be a seafloor and thecomponent 220, 320 can be positioned on the sea floor.

FIGS. 4 and 5 illustrate examples of a component including adifferential gravimeter for measuring a property of a geologicalformation. The differential gravimeter described herein appliesEinstein's theory of gravitation for determining a gravitationalpotential difference between positions within a geological formation.Each differential gravimeter of FIGS. 4 and 5 can include a tuningmechanism, such as the tuning mechanism 106 of FIG. 1. In FIG. 4, thetuning mechanism is a mechanical modulator. In FIG. 5 a, the tuningmechanism is a magnetic modulator.

FIG. 4 illustrates a schematic diagram of one example of a componentsuch as, for example, the component 110 of FIG. 1. The example componentincludes a differential gravimeter described 400 herein with amechanical modulator for compensating the frequency shift of radiationfrom a source. The differential gravimeter 400 can be a part of thewireline tool 210 of FIG. 2, or a part of themeasurement-while-drilling/logging-while-drilling tool 310 of FIG. 3. Itwill be appreciated that the differential gravimeter 400 can be a partof other downhole tools, including a coiled tubing tool, a testing tool,a completions tool, a production tool, or combinations thereof,depending on specific applications. In one embodiment, the gravimeter400 can be located above a resistivity tool that may be present on adownhole tool and be a part thereof, such as the wireline tool of FIG. 2or the MWD/LWD tool of FIG. 3. Resistivity tools of downhole tools areknown in the art.

The differential gravimeter 400 of FIG. 4 includes a source 410 mountedtoward one end of a housing 450. An absorber 420 can be mounted at theopposite end of the housing 450, generally facing the source 410, toreceive at least a portion of radiation generated by the source 410. Inone embodiment, the housing 450 is constructed as a tube or a conduit toprovide a line-of-sight contact between the source 410 and the absorber420. It will be appreciated that the source 410 and the absorber 420 canbe mounted at other positions of the housing 450 as long as theradiation emitted from the source 410 can transmit to the absorber 420.

The source 410 and the absorber 420 can include at least one isotope,for example, ⁵⁷Fe, ⁶⁷Zn, ¹⁰⁹Ag, ¹⁰⁷Ag, ¹⁰³Rh, or ¹⁸⁹Os. In oneembodiment, the source 410 and the absorber 420 include the same type ofisotope. The isotope can be in the form of, for example, a crystal platewhich can serve to absorb emission-recoil energy from photons emitted.It will be appreciated that the isotope may be in other suitable forms.For example, the source 410 and the absorber 420 can use a metal slicemade out of an isotope or a suitable alloy in which an isotope isembedded in a metal-crystal structure so that the metal slice or thealloy can serve as a whole to absorb the emission-recoil energy. In someembodiments, the source 410 and the absorber 420 can include singlecrystal(s) thermally diffused with isotope(s), metal foil(s) enrichedwith isotope(s), and/or crystal powders containing isotope(s).

The source 410 can generate radiation, for example, gamma radiation, ata desired narrow spectral line. It will be appreciated that theradiation could come from a variety of sources. For example, Table 1above lists examples of natural line widths for spectral lines emittedby example nuclear isotopes. It is to be understood that the line widthof gamma radiation is not limited to the listed line widths of Table 1.In other embodiments, radiations having frequency/energy that may bedifferent from gamma radiation may be used as long as the radiation canbe comparable to that of a gamma radiation, for example, having aspectral line width such as the natural line widths listed in Table 1.

The gamma radiation generated from the source 410 transmits to theabsorber 420 which can absorb at least a portion of the gamma radiation.In one embodiment, the housing 450 can include a tube or a conduitfilled with transmission media to reduce radiation absorption before thegamma radiation arrives at the absorber 420. For example, light gasessuch as, for example, helium or hydrogen can be used as transmissionmedia to remove relatively heavy elements such as, for example, argon.In another embodiment, the space between the source 410 and the absorber420 can be in a vacuum to reduce radiation absorption duringtransmission.

A detector can be operatively connected to the absorber 420 to detectthe absorption of gamma radiation. It will be appreciated that one ormore detectors may be employed. In the embodiment of FIG. 4, thedetector is constructed as a detector assembly 430 including atransmission detector 432 and two side-scatter detectors 434, to detectthe absorption. In other embodiments, the assembly 430 can include thetransmission detector 432 without including the side-scatter detector434. It will be appreciated that one or more transmission detectorsand/or one or more side-scatter detectors may be used in any suitablecombinations.

In the embodiment of FIG. 4, the transmission detector 432 can bepositioned in a forward direction, behind the absorber 420 relative tothe source 410, to detect reduced photon flux or a diminishing of thegamma radiation due to the absorption by the absorber 420. The forwarddirection described herein refers to the direction of radiationtravelling from a source to an absorber. In some embodiments, with theabsorption of the gamma radiation, the absorber 420 can re-emit photons,which can be detected away from the forward direction by a side-scatterdetector, such as the side-scatter detector 434. In this embodiment, twoside-scatter detectors 434 are positioned on each side of the absorber420. It is to be understood that one or more side-scatter detectors canbe positioned, for example, on one side of the absorber 420. In anotherembodiment, the absorption can be detected by one or more transmissiondetectors without using a side-scatter detector. The transmissiondetector and/or the side-scatter detector can include, for example, ascintillation crystal coupled to a photomultiplier tube, and/or asolid-state detector such as for photons.

The gamma radiation generated by the source 410 travels from the source410 to the absorber 420. For example, the frequency of the gammaradiation shifts due to the gravitational potential difference betweenthe positions of the source 410 and the absorber 420. For example, ifthe source 410 and the absorber 420 are in positions with differentgravitational potential, they encounter different time dilatation. Therespective gamma radiation, for example, nuclear spectral lines of thesource 410 and the absorber 420 are slightly frequency shifted withrespect to each other, and there would be relatively less resonantabsorption and/or re-emission compared to the situation without suchfrequency shift.

Such frequency shifts can be compensated by a tuning mechanism, such asthe mechanical modulator 440 of FIG. 4, so that the radiation can beresonantly absorbed by the absorber 420. In one embodiment, themechanical modulator 440 can be operatively connected to the source 410such that the source 410 can move relative to the absorber 420. Themovement of the source 410 can generate a frequency shift of the gammaradiation emitted therefrom. In one embodiment, the frequency shift canbe a Doppler shift which can be used to compensate the frequency shiftdue to the gravitational potential difference. Here, Doppler shiftrefers to a change in frequency of a wave for an object moving relativeto the source of the wave. For example, relative movements between asource and an absorber generate a Doppler shift, where the frequency ofradiation emitted from the source or received by the absorber can bechanged. In the embodiment of FIG. 4, the source 410 can be mounted on aplatform and driven by a piston having an eccentered wheel. It will beappreciated that the modulator 440 is not limited to the specificstructure of FIG. 4 and other types of modulators that can generaterelative movements between the source 410 and the absorber 420sufficient to induce a shift (e.g., Doppler shift) in gamma radiationfrequency may be used. The relative velocity between the source 410 andthe absorber 420 can be, for example, in a range from nm/second tomm/second. It will be appreciated that the relative velocity may varyaccording to the natural line widths of the isotopes of the source 410and/or the absorber 420. In some embodiments, a piezoelectric modulatorcan also be used to move the source and/or the absorber. It will beappreciated that the relative movements between the source and absorbercan be generated by various means. For example, a permanent magnetinside a solenoidal coil driven with an alternating current can also beused as an actuator modulator.

The source 410 and/or the absorber 420 can be moved, for example, in anoscillatory manner toward and away from each other. This movement causesa shift (e.g., Doppler shift) in photon energy and frequency.Oscillation speeds in a range, for example, 10 nm/second to 10mm/second, can be sufficient to sweep the emitted and resonantlyabsorbed spectral lines past each other. It will be appreciated thatother oscillation speeds may be suitable.

It will be appreciated that the mechanical modulator 440 can beoperatively connected to the absorber 420 to move the absorber 420relative to the source 410 to generate a shift (e.g., Doppler shift) sothat the frequency shift of the radiation due to the gravitationalpotential difference can be compensated.

It will be appreciated that the absorber 420 and the source 410 each canbe operatively connected to one or more mechanical modulators, such asthe mechanical modulator 440, and the absorber 420 and the source 410can be moved independently or simultaneously to generate relativemovements and to generate a shift (e.g., Doppler shift) such that thefrequency shift of the radiation due to the gravitational potentialdifference can be compensated.

FIG. 5 a illustrates a schematic diagram of another example of acomponent such as, for example, the component 110 of FIG. 1, thatincludes a differential gravimeter 500 described herein with a magneticmodulator for compensating the frequency shift of the radiation resultedfrom the gravitational potential difference. The differential gravimeter500 can include a source 510, an absorber 520, and a detector assembly530 including a transmission detector 532 and two side-scatter detectors534, which can be contained in a housing 550. The structure and functionof the differential gravimeter 500 can be similar to the differentialgravimeter 400 of FIG. 4, except for, for example, the magneticmodulator 540 which is used instead of a mechanical modulator.

In this embodiment, the magnetic modulator 540 includes a magnetic fieldcoil that can be operatively connected to the source 510 to generateexternal magnetic field(s). The external magnetic field(s) generated canhelp compensate the frequency shift of the gamma radiation emitted fromthe source 510 by tuning it. In some embodiments, the external magneticfield(s) can shift the spectral frequency of gamma radiation using thequantum-mechanical effect of Zeeman splitting, where interactionenergies of the couplings between the nuclear magnetic moments ofisotope atoms and the external magnetic field(s) can be modulated. Suchmagnetic modulation can split a spectral line into multiple energysub-levels and allow the energy difference of the magnetic splittings toaccount for the gravitational frequency shift so photons can beresonantly absorbed. The Zeeman splitting effect is known in the art.

In one embodiment, as shown in FIG. 5 b, the gamma radiation travellingfrom a source to an absorber has a frequency shift of(f_(absorber)−f_(source)). The spectral line of the source or absorbercan have a four-fold degeneracy with the total nuclear angular momentumj=3/2. In the presence of an external magnetic field B_(Zeeman), thesingle spectral line of the source/absorber can be split into foursub-spectral lines, as denoted by the z-component m(m=+3/2, +1/2, −1/2,−3/2) of the total angular momentum. The magnitude of the magneticsplitting can be a function of the external magnetic field B_(Zeeman).For example, when B_(Zeeman)=B_(1bias), the splitting of the spectralline with m=−3/2 can compensate the gravitational frequency shift. Asanother example, when B_(Zeeman)=B_(2bias), the splitting of thespectral line with m=−1/2 can compensate the gravitational frequencyshift.

Such external magnetic field(s) can be controlled to tune the frequencyof the gamma radiation without moving the source 510 or the absorber520. In one embodiment, the magnetic field can be generated, forexample, inside a solenoidal coil around the source 510 by an electriccurrent. The intensity of the electric current can be adjusted todetermine the magnetic-field strength. The electric current can becontrolled to have a relative precision of, for example, one part in1000 or better. For example, a 1 mA current can be controlled with aprecision of ±1 μA or better.

The magnetic field(s) generated can include a superposition of twocomponents, e.g., a baseline component and an oscillating component. Thebaseline component can be a substantially constant field that can imposea bias to shift a spectral line of the radiation by a determined amountagainst a zero-field reference. In one embodiment, the baselinecomponent can cause a splitting of a spectral line by coupling nuclearmagnetic moments to electron magnetic moments. The baseline componentcan be, for example, a few hundred Gauss or milli-Tesla. It will beappreciated that the baseline component of other magnitudes can be usedto split the spectral line.

The baseline component can serve to compensate for a majority of thefrequency shift due to the gravitational potential difference betweenthe positions of a source and an absorber, such as the source 510 andthe absorber 520. For example, if the position of the source 510 has alower gravitational potential than the position of the absorber 520, thegamma radiation travelling from the source 510 to the absorber 520 has ared shift in terms of frequency. A magnetic modulator, such as themagnetic modulator 540, can be operatively connected to the source 510and a constant baseline component of a magnetic field generated canshift the frequency of the radiation from the source 510 upward tocompensate the red shift.

The gravitational potential difference may depend on a tilt angle of thegravimeter against a gravitational vertical. To compensate the frequencyshift, the baseline component can be adjustable by, e.g., controllingelectric current, according to the tilt angle of the gravimeter againstthe gravitational vertical. In one embodiment, the tilt angle can bemeasured by a tilt meter, for example, a triaxial accelerometer, whichcan determine the angle within a resolution of, for example, about 0.1degree or better. It will be appreciated that the tilt angle can bemeasured by a tilt meter other than a triaxial accelerometer. Aprocessor operatively connected to the gravimeter 500, such as theprocessor 108 of FIG. 1, can have a module such as the module 108 ashown in FIG. 1 to determine a dependence of the baseline component onthe tilt angle.

The oscillating component of the magnetic field can then sweep thespectral line of the radiation over a frequency range to compare thesource and absorber spectral resonance frequencies. The oscillatingcomponent may be considerably weaker than the baseline component. Forexample, the oscillating component may have the amplitude of a fewpercent of the baseline component. The frequency of the oscillatingcomponent can be determined based on a given source, such as the source510, and/or a given absorber, such as the absorber 520. For example, thefrequency of the oscillating component can be around 10-100 MHz for⁵⁷Fe, and around 10-100 kHz for ⁶⁷Zn. The oscillating component mayfollow a shaped pulse, for example, a triangular or saw-tooth pulse thatcan linearly sweep the spectral frequency of the source and/or theresonant spectral frequency of the absorber so as to compare thefrequencies of the source and the absorber. It will be appreciated thatpulses other than triangular or saw-tooth shaped can also be used.

Some sources, e.g., ¹⁰⁹Ag, can emit gamma radiation having such a narrowline width that the Earth's magnetic field can split the spectral lineof gamma radiation into components. Some sources, e.g., ⁵⁷Fe and ⁶⁷Zn,have relatively wider line width that the Earth's magnetic field canbroaden the spectral line. In some embodiments, the magnetic modulatormay generate a magnetic field having a component to compensate for theEarth's magnetic field. For example, the magnetic modulator 540 mayinclude magnetic coil(s) to compensate for a component of the Earth'smagnetic field, e.g., in line with an axis of a gravimeter. In oneembodiment, the Earth's magnetic field is measured as a three-componentvector at a well site by a triaxial magnetometer. The Earth's magneticfield can be assumed to be valid in a volume around the wellsite thatincludes multiple wells drilled from the well site. A relevantmagnetic-filed component can be in line with an axis of a gravimeter.

In one embodiment, the magnetic modulator 540 is operatively connectedto the source 510 such that the magnetic field(s) generated can bearound the source 510. In other embodiments, the magnetic modulator 540can be operatively connected to the absorber 520 so that the magneticfield(s) generated can be around the absorber 520. In some embodiments,the baseline component and the oscillating component may be decoupledand separately installed around the source 510 and/or the absorber 520.

The differential gravimeter described herein can measure a gravitationalpotential, which is scalar, different from a vector-valued gravitationalforce. The differential gravimeter described herein may not have to bealigned with a direction of gravity. For example, the differentialgravimeter described herein may be used in a deviated or horizontalwellbore to monitor density changes in a geological formation, such aslateral fluid fronts replacing gas or water.

FIG. 6 illustrates a flow diagram of an example method for determining aproperty of a geological formation. It will be appreciated that themethod of FIG. 6 can be implemented in any system and/or componentdescribed in FIGS. 1-5.

At block 610, a source, such as the source 102, 410, 510, generatesradiation, for example, gamma radiation, at a first position of ageological formation, such as the geological formation 202, 302.

At block 620, the gamma radiation is emitted from the first positionthrough, for example, a housing such as the housing 450, 550, optionallyfilled with helium, to a second position of the geological formation.The gamma radiation travelling from the first position to the secondposition has a frequency shift resulted from a gravitational potentialdifference of the geological formation between the first and secondpositions.

At block 630, the frequency shift of the gamma radiation can becompensated by a tuning mechanism, such as the mechanical modulator 440,and/or the magnetic modulator 540, to induce a resonant absorption ofthe gamma radiation. It is to be understood that tuning mechanisms otherthat the mechanical modulator and the magnetic modulator can also beused to compensate the frequency shift.

At block 640, at least a portion of the gamma radiation is absorbed withthe absorber, such as the absorber 104, 420, 520, at the secondposition. The frequency shift of the gamma radiation can be compensatedsuch as by step at block 630 so that the gamma radiation can beresonantly absorbed by the absorber.

At block 650, the absorption of the gamma radiation is detected by adetector, such as the detector assembly 430, 530. In one embodiment, adiminishing of radiation intensity can be detected with a transmissiondetector such as the transmission detector 432, 532, for example, bydetecting a reduced photon flux in a forward direction after the gammaradiation is absorbed by an absorber, such as the absorber 420, 520. Inanother embodiment, photons re-emitted by an absorber upon theabsorption can be detected away from the forward direction by one ormore side-scatter detectors, such as the side-scatter detectors 434,534. The detector assembly then generates at least one signal associatedwith the frequency shift due to the gravitational potential difference.

At block 660, the gravitational potential difference can be determinedby a processor, such as the processor 230, 330, 108, from the at leastone signal generated by the detector assembly. It is to be understoodthat one or more processors can be used to determine the gravitationalpotential difference.

At block 670 the processor can determine a property of the geologicalformation from the gravitational potential difference determined. Forexample, a density of the geological formation can be determined fromthe gravitational potential difference. The conversion from agravitational potential difference determined to a density of ageological formation can be achieved, e.g., by a method that utilizes aconcentric spherical shell model, as described further below. It will beappreciated that formation properties other than the formation density,such as changes of fluid compositions, can also be determined from thegravitational potential difference.

FIG. 7 illustrates a flow diagram of one example of a method fordetermining a density of a geological formation. It will be appreciatedthat the method of FIG. 7 can be implemented in any system and/orcomponent described in FIGS. 1-5.

The method of FIG. 7 can be similar to the method of FIG. 6 except for,for example, the additional of another detecting step. At block 710,similar to the block 610, a source, such as the source 102, 410, 510,generates radiation, for example, gamma radiation, at a first positionof a geological formation, such as the geological formation 202, 302.

At block 720, similar to the block 620, the gamma radiation is emittedfrom the first position through, for example, a housing such as thehousing 450, 550 filled with helium, to a second position of thegeological formation. The gamma radiation travelling from the firstposition to the second position has a frequency shift resulted from agravitational potential difference of the geological formation betweenthe first and second positions.

At block 730, similar to the block 630, the frequency shift of the gammaradiation can be compensated by a tuning mechanism, such as themechanical modulator 440, and/or the magnetic modulator 540, to induce aresonant absorption of the gamma radiation.

At block 740, similar to the block 640, at least a portion of the gammaradiation generated by the gamma radiation at the first position isabsorbed with an absorber, such as the absorber 104, 420, 520. Thefrequency shift of the gamma radiation can be compensated such as by thestep at the block 730 so that the gamma radiation can be resonantlyabsorbed.

At block 750, the absorption of the gamma radiation is detected by adetector, such as the detector assembly 430, 530. In one embodiment, adiminishing of radiation intensity can be detected in a forwarddirection, such as at the block 650. In addition, photons re-emitted byan absorber upon the absorption can be detected away from the forwarddirection. The detector generates at least one signal associated withthe frequency shift due to the gravitational potential difference.

At block 760, similar to the block 660, the gravitational potentialdifference can be determined by at least one processor, such as theprocessor 108, 230, 330, from the at least one signal generated by thedetector assembly.

At block 770, similar to the block 670, the at least one processor candetermine a property of the geological formation from the gravitationalpotential difference determined. For example, a formation density ofgeological formation can be determined from the gravitational potentialdifference. It will be appreciated that a property of the geologicalformation is not limited to the formation density. Properties of thegeological formation other than the formation density, such as changesof fluid composition therein, can be determined from the gravitationalpotential difference determined.

FIG. 8 illustrates a schematic diagram of an example concentricspherical shell model for determining a density of a geologicalformation from a gravitational potential difference determined. In theconcentric spherical shell earth model, the earth can be represented bya generally spherical body 800 having concentric shells, e.g., theconcentric shell 810. Equation (4) gives the mass contribution dm of aconcentric shell at radius r as:dm=4πr ²ρ(r)dr  (4)where ρ(r) is a density at radius r. The total mass m of the sphere outto a radius r₀ is:

$\begin{matrix}{{m\left( r_{0} \right)} = {4\pi{\int_{0}^{r_{0}}{r^{2}{\rho(r)}{\mathbb{d}r}}}}} & (5)\end{matrix}$This mass m(r₀) gives a gravitational potential Φ(r₀) up to an arbitraryconstant C:

$\begin{matrix}{{\Phi\left( r_{0} \right)} = {\frac{{Gm}\left( r_{0} \right)}{r_{0}} + C}} & (6)\end{matrix}$where G is the gravitational constant.

Between two radii r₁ and r₂ (with r₁>r₂) the difference in thegravitational potential is:

$\begin{matrix}{{{\Phi\left( r_{1} \right)} - {\Phi\left( r_{2} \right)}} = {{\frac{G\;{m\left( r_{1} \right)}}{r_{1}} - \frac{G\;{m\left( r_{2} \right)}}{r_{2}}} = {4\pi\;{G\left( {{\frac{1}{r_{1}}{\int_{0}^{r_{1}}{r^{2}{\rho(r)}\ {\mathbb{d}r}}}} - {\frac{1}{r_{2}}{\int_{0}^{r_{2}}{r^{2}{\rho(r)}{\mathbb{d}r}}}}} \right)}}}} & (7)\end{matrix}$

The concentric spherical shell model may assume that between twoclose-by radii the density is generally a constant, corresponding to anaverage density ρ_(ave). The gravitational potential difference inEquation (7) can be simplified to:

$\begin{matrix}{{{\Phi\left( r_{1} \right)} - {\Phi\left( r_{2} \right)}} = {{4\pi\;{G\left( {{\left( {\frac{1}{r_{1}} - \frac{1}{r_{2}}} \right){\int_{0}^{r_{2}}{r^{2}{\rho(r)}\ {\mathbb{d}r}}}} + {\frac{\rho_{ave}}{r_{1}}{\int_{r_{2}}^{r_{1}}{r^{2}{\mathbb{d}r}}}}} \right)}} = {{\frac{4\pi\; G\;\rho_{ave}}{3r_{1}}\left( {r_{1}^{3} - r_{2}^{3}} \right)} - {4\pi\; G\;{m\left( r_{2} \right)}\frac{r_{1} - r_{2}}{r_{1}r_{2}}}}}} & (8)\end{matrix}$The second term

$4\pi\; G\;{m\left( r_{2} \right)}\frac{r_{1} - r_{2}}{r_{1}r_{2}}$is a tree-air gradient in terms of the gravitational potential. It willbe appreciated that a free-air gradient for Newton-gravimetry forcemeasurements has a different formula.

The vertical distance δr=r₁−r₂ can be between positions of, for example,a source and an absorber such as the source 410, 510 and the absorber420, 520, which in some examples may be the source-absorber distance ata single log station in a vertical well. In a non-vertical well, thevertical distance for example can be the source-absorber distancemultiplied by the cosine of a dip angle, e.g., a deviation angle fromthe vertical direction. The vertical distance δr can be much smallerthan the earth radius R. For example, the vertical distance δr can bearound several meters or less, e.g., about 1 to about 2 meters.

For an example application of the concentric spherical shell model on adifferential gravimeter, such as the differential gravimeter 400, 500,and/or the differential gravimeter in the component 110, to determine adensity of a geological formation from a determined gravitationalpotential difference, the vertical distance δr can be determined as avertical distance between a source and an absorber, for example, avertical distance between the source 410 and the absorber 420, or avertical distance between the source 510 and the absorber 520. It is tobe understood that the vertical distance depends on an orientation ofthe differential gravimeter. For example, a differential gravimeter canbe in a non-vertical orientation where the vertical distance between thesource and the absorber therein can be determined based on thenon-vertical orientation.

With r₁=r₂+δr=r+δr, the gravitational potential difference can become:

$\begin{matrix}{{{\Phi\left( r_{1} \right)} - {\Phi\left( r_{2} \right)}} = {{{\frac{4\pi\; G\;\rho_{ave}}{3r}\left( {\left( {r + {\delta\; r}} \right)^{2} + {\left( {r + {\delta\; r}} \right)r} + r^{2}} \right)\delta\; r} - {4\pi\;{{Gm}\left( r_{2} \right)}\frac{\delta\; r}{r^{2}}}} \cong {{4\pi\; G\;\rho_{ave}r\;\delta\; r} - {4\pi\;{{Gm}(r)}\frac{\delta\; r}{r^{2}}}}}} & (9)\end{matrix}$An average formation density ρ_(ave) between the radii r₁ and r₂ can bedetermined from the gravitational potential difference δΦ(r)=Φ(r₁)−Φ(r₂)as:

$\begin{matrix}{\rho_{ave} = {{\frac{1}{4\pi\; G\; r}\frac{{\delta\Phi}(r)}{\delta\; r}} + \frac{m(r)}{r^{3}}}} & (10)\end{matrix}$

The average formation density ρ_(ave) can be determined with an accuracyΔρ_(ave), for example, about ±0.01 g/cm³. Given the accuracy Δρ_(ave) ofabout ±0.01 g/cm³, a corresponding measurement precision for thegravitational potential difference δΦ(r) can be about 4% of ameasurement precision of the free-air gradient. In one embodiment, themeasurement precision for the gravitational potential difference δΦ(r)can be around 5.3 m²/s² or better which can correspond to a densityerror of about 0.01 g/cm³ or less.

In some embodiments, a density ρ of a geological formation, such as theabove average formation density ρ_(ave), between a first and a secondpositions such as the above r₁ and r₂, can be determined by a generallymonotonically increasing function of the gravitational potentialdifference determined. Such function can be linear or non-linear. ATaylor series can be used to represent the non-linear function. In oneembodiment, a lowest-order linear approximation of the Taylor series canbe used to determine the density ρ:

$\begin{matrix}{\rho = {{a\frac{\Delta\;\Phi}{d}} + b}} & (11)\end{matrix}$wherein ΔΦ is the gravitational potential difference determined, d is avertical distance between the first and second positions, and a and bare coefficients. A specific example of such general relationship inEquation (11) is illustrated by Equation (10) described above where thecoefficient a can be determined by

$\frac{1}{4\pi\;{GR}}$and the coefficient b can be determined by

$\frac{M}{R^{3}},$with G being the gravitational constant, R being the earth radius, and Mbeing the earth mass. It will be appreciated that the format of saidgeneral relationship between formation density and gravitationalpotential difference may vary without changing its substance.

Generally, the differential gravimetry measurement processes describedherein, including, for example, photon emission, propagation andabsorption, can operate over time intervals of a few tens to hundreds ofnanoseconds (ns), which is much faster than the typical shock spectrumof millisecond (ms) events encountered in, for example,measurement-while-drilling (MWD) or logging-while-drilling (LWD). Thedifferential gravimeters described herein may operate even during thedrilling process and in relatively harsh environments includingconditions of high temperature, pressure, shock and vibrationsencountered for example in MWD/LWD. Furthermore, the gravimeters maywell be suited for the mechanically hostile MWD/LWD environment.

The differential gravimeter described herein may also be implemented asa stationary gravimeter, either for various surface measurements, or aspermanently installed downhole gravimeter for long-duration time-lapsesurveys.

The system and method described herein may be embodied in other formswithout departing from the spirit or novel characteristics thereof. Theembodiments disclosed in this application are to be considered in allrespects as illustrative and not limitative. The scope of thedescription is indicated by the appended claims rather than by theforegoing description; and all changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

The invention claimed is:
 1. A system for determining a property of ageological formation, the system comprising: a component disposed abouta downhole tool, the component to measure a frequency shift of gammaradiation travelling from a first position of the component to a secondposition of the component, the frequency shift resulted from agravitational potential difference between the first position and thesecond position, which correspond to respective positions of thegeological formation, the component including: a source positioned atthe first position, the source to emit gamma radiation from the firstposition to the second position, an absorber positioned at the secondposition, the absorber to absorb at least a portion of the gammaradiation emitted from the source, a tuning mechanism operativelyconnected to at least one of the source and the absorber, the tuningmechanism to compensate the frequency shift and induce a resonantabsorption of the gamma radiation emitted from the source by theabsorber, and one or more detectors operatively connected to theabsorber, the detectors to detect the absorption of the gamma radiationwith the absorber and to generate at least one signal associated withthe frequency shift; and at least one processor operatively connected tothe detectors, the at least one processor to receive the at least onesignal generated, determine the gravitational potential difference fromthe at least one signal received, and determine the property of thegeological formation between the first and second positions based on thegravitational potential difference determined.
 2. The system of claim 1,wherein the tuning mechanism comprises a mechanical modulator to movethe absorber or the source relative to each other.
 3. The system ofclaim 1, wherein the tuning mechanism comprises a magnetic modulator togenerate a magnetic field around the source or the absorber to generatea magnetic splitting of the gamma radiation.
 4. The system of claim 3,wherein the magnetic field comprises a baseline component and anoscillating component.
 5. The system of claim 4, wherein the baselinecomponent is adjustable according to a tilt angle of the componentagainst a gravitational vertical.
 6. The system of claim 4, wherein thebaseline component is adjustable to compensate for an earth magneticfield.
 7. The system of claim 1, wherein the one or more detectorscomprise at least one transmission detector to detect a reduced photonflux due to the absorption of the gamma radiation with the absorberand/or at least one side-scatter detector to detect a re-emitted photonflux away from a forward direction of the gamma radiation.
 8. The systemof claim 1, wherein the at least one processor comprises a module todetermine a dependence of the gravitational potential difference on anorientation of the component with respect to a gravitational vertical.9. The system of claim 1, wherein the downhole tool further comprises ahousing to contain the component, the housing includes helium to reduceabsorption of the gamma radiation before absorption by the absorber. 10.The system of claim 1, wherein the downhole tool comprises a wirelinetool, a measurement-while-drilling tool, a logging-while-drilling tool,a coiled tubing tool, a testing tool, a completions tool, a productiontool, or combinations thereof.
 11. The system of claim 1, wherein thesource and the absorber each include at least one isotope of ⁵⁷Fe, ⁶⁷Zn,¹⁰⁹Ag, ¹⁰⁷Ag, ¹⁰³Rh, and ¹⁸⁹Os.
 12. A method for determining a propertyof a geological formation, the method comprising: generating gammaradiation at a first position of the geological formation; emitting thegamma radiation from the first position to a second position of thegeological formation, the gamma radiation travelling from the firstposition to the second position having a frequency shift resulted from agravitational potential difference of the geological formation betweenthe first and second positions; compensating the frequency shift of thegamma radiation to induce a resonant absorption of the gamma radiation;absorbing at least a portion of the gamma radiation at the secondposition of the geological formation; detecting the absorption of thegamma radiation and generating one or more signals associated with thefrequency shift; determining the gravitational potential differencebased on the one or more signals generated; and determining the propertyof the geological formation between the first and second positions basedon the gravitational potential difference determined.
 13. The method ofclaim 12, wherein compensating the frequency shift comprises generatinga mechanical movement of a source and/or an absorber.
 14. The method ofclaim 12, wherein compensating the frequency shift comprises generatinga magnetic field around a source and/or an absorber to generate amagnetic splitting of the gamma radiation.
 15. The method of claim 14,wherein the magnetic field comprises a baseline component and anoscillating component.
 16. The method of claim 15, wherein the baselinecomponent is adjustable according to a tilt angle with respect to agravitational vertical.
 17. The method of claim 12, wherein detectingthe absorption comprises detecting a reduced photon flux in a forwarddirection of the gamma radiation.
 18. The method of claim 12, whereindetecting the absorption comprises detecting a re-emitted photon fluxaway from the forward direction of the gamma radiation.
 19. The methodof claim 12, wherein the gamma radiation is generated or absorbed by atleast one isotope of ⁵⁷Fe, ⁶⁷Zn, ¹⁰⁹Ag, ¹⁰⁷Ag, ¹⁰³ Rh, and ¹⁸⁹Os. 20.The method of claim 12, wherein the property of the geological formationcomprises a formation density of the geological formation.